The different magnitudes of stress throughout an object are known as residual stress. The name comes primarily from the fact that residual stress is the stress remaining within an object as a result of service induced overloads, forming, shaping or other processing that changes the internal residual stress of an object. Objects are generally formed by exposure to a change in energy, heat, or an application of pressure. Whenever an object is exposed to such a change or local deformation occurs, residual stresses may change. This change in residual stress effects the atoms within the object by increasing or decreasing the spatial configuration between neighboring atoms.
Quantifying the residual stresses present in a component, which may either accelerate or arrest fatigue, fracture, distortion, wear, creep, or stress corrosion cracking, is frequently crucial to understanding a cause of failure in a component part. Current methods of measuring residual stress calculate the surface changes of a pre-existing surface. Because the measurements are indirect representations of displacement at the surface, the residual stress calculations are theoretically complex and tedious, and can only be performed on nonplated and noncoated parts. For example, in some methods, a material is subjected to radiation and a series of resulting diffraction peaks are measured to determine the distance between atoms and/or lattice planes. Strength related characteristics, such as stress, retained austenite, hardness, level of fatigue, etc can affect this measurement. Diffraction methods alone may be limited to measuring across a large number of positions on the material to obtain unrevealing information, particularly where the material being tested has been used in the field where corrosion and other environmental use conditions can cause highly localized variations in the strength characteristics being determined. When the only measurements taken include such localized aberrations, the determination of residual stress within a material can be effected.
Furthermore, it may also be desirable to quantify residual stress of a metallic material having layers applied to the pre-existing surface, including, for example, layers of protective coating, corrosive resistant plating, paint and primer.